Particle detection and analysis is desirable in a variety of manufacturing and environmental contexts. For example, in clean rooms used for the fabrication of integrated circuits, highly accurate particle detection is required due to the small dimensions of the devices under production. Significant failure rates in integrated circuits are associated with the presence of particles greater than one-tenth the device linewidth. Typically, the smaller the size of the particle, the greater the number of particles which are present. Therefore, as linewidths decrease within the sub-micron range, particle removal becomes increasingly difficult and costly. Consequently, control of a particle source is usually more cost-effective than removal of particles once they are liberated from their source. Through real-time particle analysis, particle sources can be identified and controlled.
Particle detection and analysis in clean rooms and gas distribution systems is typically done by real-time, also known as on-line, counting of airborne particles followed by off-line analysis of deposited particles by microscopic or laser scan techniques. The former technique provides the rapid response required for monitoring particle generation events while the latter technique provides size and elemental composition information. Particle counting is frequently performed using standard light-scattering particle counters. However, these devices can only detect particles on the order of 50 nanometers or greater and provide no compositional information. While off-line analysis provides particle composition information, it is also limited by the particle sizes it can detect and cannot be time-correlated to particle generation events.
Mass spectrometry is an analytical technique used for the accurate determination of molecular weights, identification of chemical structures, determination of mixture compositions, and quantitative elemental analysis. Molecular structure is typically determined from the fragmentation pattern of ions formed when the molecule is ionized. Elemental content of molecules is determined from mass values obtained using mass spectrometers. However, since mass spectrometers typically operate in vacuum, particulate analysis usually requires that nearly all of the particulate carrier be separated from the particulate material prior to ionization in the spectrometer. This requirement increases the complexity of particle detection for particulates suspended in liquids and gases.
Real-time or on-line particle analysis for particles suspended in gases is normally accomplished by sampling particles through a differentially pumped nozzle and impacting the particle beam onto a heated surface. In this manner, impinging particles are ionized and analyzed. However, this surface ionization technique results in the creation of ions from both the particle beam and the surface being heated, making it difficult to determine the composition and size of the particles of interest. Additionally, not all elements of the particulate sample will form ions, resulting in discrimination against certain elements, typically those elements with high electronegativities and high ionization potentials.
More universal detection can be achieved through electron impact ionization of neutral species ejected by the collision of a particle beam with a heated surface. However, this method creates extensive fragmentation and results in lower ionization yields than surface ionization. Scanning mass analyzers, such as the quadrapole or magnetic sector analyzers can also be used for particulate analysis. Due to the transient nature of the signal produced in these devices, it is difficult or impossible to obtain a complete mass spectrum. As a result, these analyzers show poor sensitivity and difficulty in performing multicomponent determinations.
Many of the difficulties associated with the above techniques can be reduced or eliminated through the use of a laser-induced mass spectrometry system taught in U.S. Pat. No. 5,382,794 issued Jan. 17, 1995, commonly assigned to the instant assignee, the disclosure of which is incorporated by reference herein. In the patent, an exemplary laser-induced mass spectrometry system is described in which particles enter an evacuable chamber through an inlet device such as a capillary. A laser, such as a pulsed laser, is positioned such that the laser beam intersects the particle stream. As the particles pass through the path of the laser beam, they are fragmented and ionized. A detector, such as a time-of-flight mass spectrometer detects the ionized species. Mass spectra are produced, typically being recorded with an oscilloscope, and analyzed with a microprocessor. The mass spectra information permits real-time analysis of the particle size and composition.
While the laser-assisted spectrometry system described in the patent provides useful real-time particulate analysis, there is a continuing need to provide compositional and size evaluation for increasingly smaller particulates. There is a further need in the art for detection and analysis of a greater percentage of the particulate contents of a sample, to ensure accurate characterization. Finally, there is a need in the art for particulate analysis systems and techniques which do not discriminate against high electronegativity and high ionization potential elements.